The present invention relates to a vapor deposition method for the GaAs thin film by which it has been made possible particularly to manufacture the wafer with a small dispersion in the distribution of the carrier density of the thin film of deposited n-type conductive crystals.
In general, for the vapor phase deposition for the GaAs thin film, a vapor deposition apparatus shown in FIG. 6 is used, wherein a high-frequency induction heating coil (5) is provided around the outer circumference of a reactor (1) with an introductory port (2) of source gases at the upper end and an exhaust port (3) of gases at the lower end through a cooling jacket (4) and a carbon susceptor (6) in the shape of truncated hexagonal pyramid is arranged in the reactor (1). With this apparatus, GaAs substrates (7) are fitted onto the pyramid faces of the susceptor (6) and, allowing the susceptor (6) to rotate in the direction of arrow mark, source gases are introduced into the reactor (1) from the introductory port (2) and allowed to eject from the exhaust port (3) at the lower end. In this way, the substrates (7) are heated to a predetermined temperature by the heating coil (5) and, through the thermal decomposition of source gases near the surface of the substrates (7), the crystals of GaAs are allowed to deposit onto the substrates (7).
As the source gases, organic gallium gas, for example, trimethyl gallium (Ga(CH.sub.3).sub.3) and arsine gas (AsH.sub.3) are used. In general MOCVD method (Metal Organic Chemical Vapor Deposition), the type of conduction and the carrier density of deposited crystals when the addition of impurity is not made intentionally depend on the supplying ratio (V/III) of Ga(CH.sub.3).sub.3 (III) and AsH.sub.3 (V) as shown in FIG. 7. In the growth at atmospheric pressure of the reactor, if V/III is smaller than 10 to 20, p-type conductive crystals are produced, if it is larger than 10 to 20, n-type conductive crystals are produced and, if it is between 10 and 20, high resistive crystals with the lowest carrier density are produced (solid line). In the growth of reduced pressure (for example at 100 Torr), the value of V/III where p-type conductive crystals change to n-conductive crystals is larger than atmospheric pressure and between 20 and 50 (dashed line). Moreover, through the addition of gas having an impurity possible to become the source for the formation of electrons as an ingredient element to both source gases, the impurity is added to the depositing GaAs crystals to produce n-type conductive crystal film. As the impurity, sulfur is used most frequently and, as the gas containing this, hydrogen sulfide gas (H.sub.2 S) is used. By varying the flow rate of this gas, the concentration of electrons, that is, the carrier density of n-type crystals is controlled.
In this way, for the deposition of, for example, epitaxial wafer used for FET, high resistive crystal film (hereinafter abbreviated as buffer layer) having a thickness of 2 to 3 .mu.m is allowed to deposit onto the GaAs substrate and n-type conductive crystal film (hereinafter abbreviated as doped layer) having a thickness of about 0.5 .mu.m is allowed to deposit thereon. Using non-doped crystals added no impurity intentionally for the buffer layer making the supplying ratio (V/III) of source gases 10 to 20 at atmospheric pressure and 20 to 50 at reduced pressure (for example at 100 Torr) so as to make the resistance highest among the crystals by general MOCVD method and heating the substrates at 600.degree. to 700.degree. C., the buffer layer is deposited and, in succession, by adding hydrogen sulfide gas to source gases, the doping layer is deposited.
Although the carrier density of the depositing doped layer can be controlled by the flow rate of H.sub.2 S, it depends also on other deposition conditions. Namely, the carrier density varys also with the supplying ratio (V/III) of AsH.sub.3 and Ga(CH.sub.3).sub.3 and further with the flow rate of Ga(CH.sub.3).sub.3 or the temperature of susceptor. As a result, in the crystal deposition of the wafer with a large area, there has been a problem that the dispersion is caused in the distribution of the carrier density of the doped layer of wafer due to the temperature distribution of susceptor, the difference in the decomposition ratio of AsH.sub.3 resulting from the location on susceptor, or the like. For example, by conventional method before mentioned, the dispersion of the carrier density in a wafer were 6 to 7%.